In general, multilayer ceramic electronic components, such as multilayer ceramic capacitors, have been manufactured, for example, by forming dielectric ceramic green sheets from ceramic slurry mainly composed of a powdery dielectric material, such as barium titanate, printing electrodes on the ceramic green sheets, punching the ceramic green sheets into a predetermined shape, and stacking the punched sheets.
Unnecessary parts remaining after the ceramic green sheets have been punched may be reused as the ceramic raw material. However, the unnecessary parts often cannot be reused because the nonuniform particle size distribution of redispersed ceramic raw material may cause variations in the dielectric properties after firing, or electrode components remaining on the ceramic green sheets may have adverse effects on the dielectric properties as impurities.
Thus, effective reuse of waste titanate ceramic materials mainly composed of barium titanate has been investigated.
A carbon dioxide absorbent containing at least one selected from the group consisting of lithium silicates having a general formula of LixSiyOz has been proposed as a carbon dioxide absorbent for recovering carbon dioxide (CO2), emitted from power plants using hydrocarbon fuels and automobiles, at high temperatures (see Patent Document 1).
This carbon dioxide absorbent is lightweight and absorbs carbon dioxide at a temperature of more than about 500° C.
More specifically, lithium silicate (Li4SiO4) absorbs carbon dioxide at a high temperature of more than 500° C. in the reaction represented by (1).Li4SiO4+CO2→Li2SiO3+Li2CO3  (1)
However, the volume of lithium silicate varies greatly with the absorption and desorbtion of carbon dioxide. Thus, the strength of the absorbent decreases gradually owing to repetitive stress over time. More specifically, absorption of carbon dioxide increases the volume of lithium silicate by about 1.4 times. Thus, the repetitive absorption and desorbtion of carbon dioxide reduces the strength of the absorbent to finally disintegrate the absorbent. Hence, an absorbent made of lithium silicate has low durability.
While lithium silicate is intended to absorb carbon dioxide in a high-temperature combustion gas, it is actually difficult to remove carbon dioxide from exhaust gas having a high temperature of 700° C. or more.
The use of lithium silicate is also examined to separate carbon dioxide before combustion in a hydrogen production process, for example, used for fuel cells. However, reactions occur at a temperature of at least 700° C. in an actual process of hydrogen production including steam reforming of natural gas. Thus, it is difficult to remove carbon dioxide with lithium silicate at such a high temperature.
Furthermore, the composition of the absorbent becomes nonuniform over time because lithium carbonate becomes liquid at high temperature.
In such a situation, a method for absorbing carbon dioxide with Ba2TiO4 has been proposed (prior application 1 (Japanese patent application No. 2004-348990)). Ba2TiO4 is prepared by firing barium titanate (BaTiO3) in the presence of barium carbonate (BaCO3) according to the chemical reaction (2).BaTiO3+BaCO3→Ba2TiO4+CO2↑  (2)
Ba2TiO4 absorbs carbon dioxide to yield BaTiO3 according to the chemical reaction (3) under particular conditions.Ba2TiO4+CO2→BaTiO3+BaCO3  (3)
The resulting BaTiO3 desorbs carbon dioxide and reverts to being Ba2TiO4 according to the reaction (2) at a particular pressure (reduced pressure of 1000 Pa or less) and a particular temperature (at least 750° C.).BaTiO3+BaCO3→Ba2TiO4+CO2↑  (2)
Thus, according to the chemical reactions (3) and (2), the carbon dioxide absorbent Ba2TiO4 can be used to absorb carbon dioxide at high temperatures and can be regenerated by desorbing carbon dioxide under particular conditions. The carbon dioxide absorbent Ba2TiO4 can therefore be used repeatedly to absorb carbon dioxide efficiently.
However, there is a demand for a carbon dioxide absorbent that can absorb and desorb carbon dioxide at a temperature lower than that of the absorbent mainly composed of Ba2TiO4, or a carbon dioxide absorbent that can absorb more carbon dioxide than the absorbent mainly composed of Ba2TiO4 to improve the flexibility of operating conditions and the absorption and desorbtion efficiency of carbon dioxide.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-262890